The objective of this study is to develop and test a continuous monitor for the identification and counting of asbestos fibers in ambient air and in occupational and residential environment. The instrument determines the size and asymmetry of a dilute suspension of particles large compared with the wave length of incident light. The particles pass through a rotating electric field. The particles align themselves to the electric field and will present a varying scattering cross section as seen by a photoelectric detector, depending on the instantaneous orientation of the electric field. The resulting output from the detector will be analyzed using a high speed computer to determine the total particle count in a number of size and shape categories.